The present invention relates to a method of making dental replacements, in particular inlays, onlays, and crowns, and to an apparatus for implementing said method, wherein a template is made, the template is scanned by means of at least one sensor, and the scanning movements are transmitted to at least one processing tool, so that a dental replacement whose shape corresponds to that of the template can be made from a blank in the form of a substrate mass.
Most dental fillings are at present made of amalgam. The reasons are their long life, low cost, and the fact that they can be made and fitted in a single session of the patient's at the dentist's. The main disadvantages are the low aesthetic worth of amalgam fillings and the fact that the health aspects of the materials used in their manufacture are no longer undisputed.
The known alternatives are gold fillings, composites, inlays, and onlays. Of these, composites are suitable only for fairly small replacements. Gold fillings or inlays are available for larger replacements. Because metal fillings are expensive and their colour cannot be matched with that of the natural tooth, they are being increasingly superseded by inlays and onlays made of resistant synthetic or ceramic materials whose colour can be matched with that of the tooth. They are produced by means of a template made in the patient's mouth. The classic procedure for the production of the so-called dental technician's ceramic inlays and onlays is to make a negative template. The inlay or onlay itself is made by means of this negative template, by a series of steps in which it is fitted. The accuracy of fit of these dental technician's inlays and onlays can be improved by increasing the number of steps of firing. This method is expensive and, in particular, requires the patient to have two sessions at the dentist's. Because of this, other methods have been developed that attempt to make it possible to produce an inlay or onlay filling in a single session. The factor common to all these methods is that they make use of a mobile processing tool that produces, the inlay or onlay directly from a blank by cutting milling, and grinding.
EP-B-0 182 098 describes such a method, in which the information for the control of the processing tool is computed from an image of the prepared tooth projected on a monitor and by manual input of the section line on the monitor. After data acquisition, the processing tool makes use of the computed data to shape the inlay or onlay by combined movements. An all three axes. This method permits inlay fillings to be made relatively quickly. But the accuracy of fit is unsatisfactory for several basic reasons. Firstly, local optical resolution and the imaging properties of the projected image cannot be of the required quality because the size of the device is necessarily small. Secondly, the connection of the manually input coordinates for the shape of the tooth is only approximate and can lead to errors in the computation of the outline-of the side walls in the prepared tooth. A further source of error is the manual input of the section line itself, because it leaves a large margin of discretion to the operator. Also, because optical means are used to measure the prepared cavity, the shape of the cavity is subject to considerable restraints, it demands the removal of unnecessary amounts of healthy dental material during preparation, and requires the dentist to learn and observe special procedures in preparing the cavity. A particular disadvantage of this method has proved to be the fact that especially large errors occur precisely in the least accessible places. Further, this method does not permit the direct shaping of the tooth's masticatory surface. Finally, the equipment based on this method is complex to make and very expensive.
EP-A-0 402 720 describes another method. As in the case of the dental technician's inlay/onlay, this makes use of a template prepared in the patient's mouth. The template is held in a movable mount and scanned by a manually controlled, likewise movable scanning head, until it has touched all the points on the template's surface. The mechanical connection of the processing tool to the scanning head is such that it copies the scanning head's movements. For the visualization of the places that the scanning head has touched, a dye that reacts to contact is applied to the template. In theory, when sufficiently fine processing tools are used in this method, it permits the production of inlays and onlays that fit accurately. In practice, this method has many drawbacks. While manual scanning is performed, the milling tool must at the same time be guided through the material of the substrate mass. The amount of force that this demands causes the loss of the requisite sensitivity for a precise scan of the template. For the scanning process, the template must be held in a somewhat unstable manner between pointed tips. Apart from deformations due to this type of support, cumulative errors occur in the pointed tips, the points of contact between the tips and the template, and in the template itself, because of inadequate control of the contact pressure used in scanning. Particularly when large amounts of material have to be removed, slight prominences and depressions in the surface form and texture become blurred. Unless special care is taken to fit the temporary inlay in the mount or if the shape of the template does not allow the mount to provide adequate support, the temporary inlay used as a template may shift between the tips of the mount that hold it in place.
To keep errors due to elastic deformation as small as possible when a pointed tool is used, the template must be made of hard material. But a template of hard material is more difficult to secure in the machine. The most serious difficulties due to the use of hard filling material are the problems that arise when the template is removed from the tooth. For example, because it closely fits the tooth, the temporary inlay is difficult to remove. If there is undercutting in the preparation of the cavity, removal of the complete temporary inlay becomes impossible without permanent damage to the tooth, and the tooth has to be prepared again under aggravated conditions.
In the apparatus based on this method, the scanning head and the processing tool must have a large number of degrees of freedom to ensure the requisite facility of use and control. This is a serious disadvantage, because it requires a very complex mechanical construction that must at the same time meet high standards of accuracy. Further, because of the mutual effects of the various degrees of freedom, simple means of adjustment are no longer adequate to compensate tool tolerances in such an instrument. This results in further inaccuracies attributable to tool changes during processing.
In this method, the removal of the temporary inlay from the tooth, the method of shaping the dental replacement, the manner in which the template is fitted in its mount, and the scanning process itself all demand a high degree of skill in the operator. The results are subject to considerable quality fluctuations and often demand a very large amount of time.
Another known method, similar to that described above, is described in EP-B-0 267 227. This makes use of an automatic scanning process similar to that used in profile-milling machines. For this purpose, a hydraulic valve is switched by surface contact and controls a drive system that moves the hydraulic valve and the processing tool in accordance with the surface profile. This arrangement does not meet the required standards of accuracy and is not suitable for practical use in dentistry. Inaccuracies occur in the operation of the hydraulic valve, because static friction of the movable tip requires the application of excessive force for scanning. To eliminate static friction, it is suggested that the movable tip be set to rotate. This is technically impossible because of the mobility required and would reduce by only an insignificant amount the force required at the movable tip, because, when the surface to be scanned rotates, the forces that occur act mainly tangentially upon the tip and thus cause friction between the movable tip and the walls of the drilled guide hole. In addition, the rotating tip could easily damage the original. Finally, because of the pressures that typically occur in hydraulic systems, only a press fit or special se&ling systems can prevent the hydraulic fluid leaking from at the hydraulic valve. Either of these produces friction and hence require still more force to move the tip.
Moreover, the operation of this type of hydraulic valve is subject to relatively large hysteresis. Also, the operation of hydraulic valves is extremely progressive and the force required to move the processing tool is not very great. This results in a strong tendency for the control loop to vibrate, which can be controlled only by appropriate damping and thus requires greater force to operate the valve. Further, hysteresis as such produces further inaccuracies. Finally, use of a hydraulic system makes the system described extremely complex and expensive.
The use of gears cannot achieve a permanently precise, smooth synchronous rotation of original and copy. Errors due to slip, slack, and rough running of the gears cumulate with those of the hydraulic valve and hydraulic system.
The required angle for proper functioning, between the tip and the processing tool on the one hand and the axis of rotation of the original and the copy on the other, calls for a sharply pointed processing tool of small circumference. The speed at the processing tool's center of rotation is zero. At the high running speeds required for this type of processing tool, the tool's useful life in this type of arrangement is not long enough for the production of a single inlay, for example of ceramic material. In particular, the lack of a means of supplying cooling lubricant makes it impossible to process hard materials or remove large amounts of material.
The angle referred to above also limits the usefulness of this arrangement, particularly for the production of inlays. Thus, for example, the side wall of an inlay and the steep sides of a box-type cavity preparation remain inaccessible for the tip and the processing tool.
In the arrangement described in the publication last referred to, the processing tool and the sensor head may have different shapes. Differences of shape in the processing tool and the sensor head reduce copying accuracy, particularly when shapes have a distinct surface form or texture, as in the case of an inlay. In practice, the high speed of rotation of the shaped mass, at about 1'000 rpm, makes it impossible to produce distinct forms or textures because of the finite acceleration of the scanning and process-tracking system. In particular, the inertia of the sensor's mass limits to very low values the maximum acceleration that the scanning and process-tracking systems can achieve.